Re: [RFC PATCH 13/19] x86/resctrl: Add PLZA state tracking and context switch handling
From: Babu Moger
Date: Tue Feb 17 2026 - 11:39:27 EST
Hi Ben,
On 2/17/26 09:56, Ben Horgan wrote:
Hi Babu,
On 2/16/26 22:52, Moger, Babu wrote:
Hi Ben,Sorry, I wasn't clear. What I'm trying to understand is what you gain by
On 2/16/2026 9:41 AM, Ben Horgan wrote:
Hi Babu, Reinette,Because hypervisor runs at CPL0, there could be some use case. I have
On 2/14/26 00:10, Reinette Chatre wrote:
Hi Babu,Does this have any effect on hypervisors?
On 2/13/26 8:37 AM, Moger, Babu wrote:
Hi Reinette,
On 2/10/2026 10:17 AM, Reinette Chatre wrote:
Hi Babu,There can be only one PLZA configuration in a system. The values in
On 1/28/26 9:44 AM, Moger, Babu wrote:
Why limit it to one CTRL_MON group and why not support it for MON
On 1/28/2026 11:41 AM, Moger, Babu wrote:
On Wed, Jan 28, 2026 at 10:01:39AM -0600, Moger, Babu wrote:Yes. Correct.
On 1/27/2026 4:30 PM, Luck, Tony wrote:Babu,
I've read a bit more of the code now and I think I understand more.
Some useful additions to your explanation.
1) Only one CTRL group can be marked as PLZA
groups?
the MSR_IA32_PQR_PLZA_ASSOC register (RMID, RMID_EN, CLOSID,
CLOSID_EN) must be identical across all logical processors. The only
field that may differ is PLZA_EN.
not completely understood that part.
oh ok.ah - this is a significant part that I missed. Since this is a per-I also missed that.
CPU register it seems
to have the ability for expanded use in the future where differentYes, MPAM has a per-cpu register MPAM1_EL1.
CLOSID and RMID may be
written to it? Is PLZA leaving room for such future enhancement or
does the spec contain
the text that state "The values in the MSR_IA32_PQR_PLZA_ASSOC
register (RMID, RMID_EN,
CLOSID, CLOSID_EN) must be identical across all logical processors."?
That is, "forever
and always"?
If I understand correctly MPAM could have different PARTID and PMG
for kernel use so we
need to consider these different architectural behaviors.
I didn't understand this question. There are 16 CLOSIDs and 1024 RMIDs.Given that CLOSID and RMID are fixed once in the PLZA configurationI was initially unsure which RMID should be used when PLZA is
enabled on MON groups.
After re-evaluating, enabling PLZA on MON groups is still feasible:
1. Only one group in the system can have PLZA enabled.
2. If PLZA is enabled on CTRL_MON group then we cannot enable PLZA
on MON group.
3. If PLZA is enabled on the CTRL_MON group, then the CLOSID and
RMID of the CTRL_MON group can be written.
4. If PLZA is enabled on a MON group, then the CLOSID of the
CTRL_MON group can be used, while the RMID of the MON group can be
written.
could this be simplified by just assuming they have the values of the
default group, CLOSID=0 and RMID=0 and let the user base there
configuration on that?
We can use any one of these to enable PLZA. It is not fixed in that sense.
this flexibility. Given that the values CLOSID and the RMID are just
identifiers within the hardware and have only the meaning they are given
by the grouping and controls/monitors set up by resctrl (or any other
software interface) would you lose anything by just saying the PLZA
group has CLOSID=0 and RMID=0. Is there value in changing the PLZA
CLOSID and RMID or can the same effect happen by just changing the
resctrl configuration?
I was also wondering if using the default group this way would mean that
you wouldn't need to reserve the group for only kernel use.
Yes, that is an option, but it becomes too restrictive. Would this approach work for the ARM implementation?
If a user wants to keep a selective set of tasks running at different allocation levels, they would need to create another new group, move all tasks from default group to new group, and leave only the selected tasks in the default group.
And if that group is later deleted, all tasks will automatically return to the default group.
Thanks,
Babu